Since lithium secondary batteries were commercialized by Sony Corporation (Japan) in 1992, there has been an increasing demand on lithium secondary batteries along with the development of portable electronic instruments, such as cellular phones, digital cameras, notebook computers, or the like, for about 20 years. Until now, lithium secondary batteries have been used as important power sources for such electronic instruments.
More recently, the application range of lithium secondary batteries has been increased, and thus they have been used not only as power sources for charging small household appliances, such as cleaners or electrically powered tools, but also as medium-capacity batteries developed so as to be applied to electric bicycles or electric scooters.
In addition, lithium secondary batteries have been also used as power sources for electric vehicles, hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), various types of robots or medium- or large-scale electric storage systems (ESS). There has been a rapidly increasing demand on such lithium secondary batteries.
Recently, as a cathode active material for such lithium secondary batteries for use in large-scale electric storage systems, a layered lithium nickel manganese cobalt oxide (LiCoxNiyMnzO2) and spinel-structured lithium manganese oxide (LiMn2O4) have been mostly utilized.
Particularly, a spinel-structured lithium manganese oxide is prepared at low cost and thus has higher cost competitiveness as compared to the other materials. In addition, in spinel-structured lithium manganese oxide, lithium ions can move rapidly via three-dimensionally-interconnected channels, thereby providing excellent high-rate performance. However, in the case of the spinel-structured lithium manganese oxide, unstable Mn3+ which has a high-spin d4 electronic configuration, is formed when the cathode is subject to discharge and the average oxidation number of manganese reaches +3.5 or less. As a result, from this, so-called, Jahn-Teller effect, the lithium manganese oxide becomes structurally unstable. Moreover, at elevated temperature, Mn3+ may suffer from a dissolution phenomenon (Mn3+→Mn4++Mn2+) in which Mn2+ produced by a disproportionation reaction or the like is dissolved into an electrolyte, resulting in gradual capacity fading of a lithium secondary battery. In addition, corrosion of manganese may occur due to the hydrofluoric acid (HF) produced by the decomposition of an electrolyte (LiPF6) under high-voltage environment during the charge and discharge, resulting in structural destruction of a cathode. Further, the dissolved manganese ions form a thick solid electrolyte interface (SEI) layer on the negative electrode, thereby increasing impedance and decreasing Coulombic efficiency.
To solve the above-mentioned problems occurring in cathode active materials including a spinel-structured manganese oxide, numbers of literatures report that a lithium manganese oxide material is doped with a small amount of at least one metal species selected from Al, Mg, Ni, Zr, Cr and so on. In this manner, a surface of a high chemical stability is formed and the average oxidation number of manganese is increased so as to inhibit the structural instability caused by formation of Mn3+ leading to Jahn-Teller distortion and dissolution of Mn2+. Meanwhile, in order to solve the problem of manganese dissolution on the surface, the surface of a lithium manganese oxide material is coated with a corrosion-resistant metal oxide, metal fluoride or metal phosphate in nano-scale. Particularly, various surface modification processes, such as a sol-gel process, spray coating process or fluid suspension coating process, are developed to form a nano-scale layer of a metal oxide, such as Al2O3, MgO or ZrO2, or AlF3 or AlPO4 on the surface of an electrode or inside an active material with a gradient in its concentration. Those methods are successfully applied commercially.
However, when modifying an active material by using the coating process developed to date according to the related literatures, an electrochemically inactive material is added to the surface of active material so that the capacity thereof is naturally decreased. On the contrary, degradation of capacity or excessive impedance may occur. Particularly, in the case of a sol-gel process, complicated processing steps are required. Therefore, there is a need for a surface treatment process by which desired functions are obtained through more simple processing steps.